It has been established by both human and animal studies that neurons in region MT/V5 are sensitive to direction and speed of motion (Britten, Newsome, Shadlen, Celebrini, & Movshon, 1996; Maunsell & Van Essen, 1983; Orban et al., 1995). A quest of auditory studies has been to find out whether sub-cortical or cortical auditory fields show a similar sensitivity.
Spatial motion coding-animal physiology. Recording from the inferior colliculus of anesthetized guinea pigs, Ingham et al. (2001) showed that, although some neurons preferred different motion direction (clockwise or anti-clockwise) relative to each other, there was no clear evidence that neurons were selective to a particular motion direction or velocity. Their results
suggest not a motion-sensitive system, but rather a coding of motion through adaptation. Specifically, neurons showed high levels of activity for the first sound of the moving sequence, and much less for the subsequent sounds. When the time between the sounds was increased, neurons showed an enhanced response for the sounds that followed the first in the sequence, consistent with the recovery from adaptation. This is against the notion that neurons are motion-sensitive. However, as the authors note, these results could be due to the fact that they did not use continuous motion.
Other studies have found neurons in primary field AI (Poirier, Jiang, Lepore, & Guillemot, 1997) and in an anterior non-primary field (AAF) (Jiang,
Lepore, Poirier, & Guillemot, 2000) of cats, which appear to be sensitive to a specific motion direction. These results are surprising, as one might expect PAF to be sensitive to motion, because it has been found to be sensitive to static localization. Furthermore, tentative evidence for motion sensitivity comes from a study on awake monkeys that were presented with sounds varying in location showed that response of AI neurons to a sound was influenced by the location of the preceding sound (Malone, Scott, & Semple, 2002). What is more, neurons showed preference for particular directions.
In summary, it is still not clear whether motion is computed by comparing ‘static’ snapshots of sounds at different location, or whether there are actually neurons that are sensitive to dynamic features of space, such as motion direction and velocity, just as in the visual system (Middlebrooks, Harrington, Macpherson, & Stecker, 2008).
Spatial motion coding-human neuroimaging. Some fMRI studies have examined motion coding by either presenting sound sequences that contain discrete shifts in spatial location, (Krumbholz et al., 2007; Krumbholz, Schonwiesner et al., 2005; Krumbholz, Schönwiesner et al., 2005), or
continuous motion ( Baumgart, Gaschler-Markefski, Woldorff, Heinze, & Scheich, 1999; Pavani, Macaluso, Warren, Driver, & Griffiths, 2002)
Different studies have used different cues to create the percept of motion; such as ITDs (Krumbholz et al., 2007; Krumbholz, Schonwiesner et al., 2005; Krumbholz, Schönwiesner et al., 2005) and ILDs (Baumgart et al., 1999; Griffiths & Green, 1999; Griffiths, Green, Rees, & Rees, 2000; H.C. Hart et al., 2004) and head-related transfer functions (HRTFs) which includes
presented over headphones (Pavani et al., 2002; Warren, Zielinski, Green, Rauschecker, & Griffiths, 2002). To localize a response to moving sounds, neuroimaging studies typically compare a condition in which the sounds are moving, with a condition that the sounds are stationary, presented at the midline (Krumbholz et al., 2007) or at various locations (Poirier et al., 2005)
Krumbholz, Schonwiesner et al. (2005) examined which auditory regions are involved in the localization of static sounds and which in auditory motion processing. To investigate the processing of static lateralized sounds, monoaurally presented sounds (only left and only right) were contrasted with diotic sounds (identical in both ear, perceived in the midline). This contrast showed activation in inferior colliculus, medial geniculate body, primary auditory cortex, and anterior PT bilaterally (Figure 2.8, red). On the contrary, when contrasting moving sounds (varying ITD from -1000 µs to 1000 µs) with stationary diotic sounds (ITD=0µs), there was a response in PT, extending to the TPJ (Figure 2.8, blue). That is, there was a relatively clear segregation of the response whereby static location processing engaged earlier sub-cortical and primary cortical regions, while motion processing engaged non-primary regions. The authors speculate that a reduction in the response for the stationary sounds in PT occurred because neurons in non-primary auditory cortical regions produce mainly a phasic, rapidly adapting response, i.e. they respond only to change. On the other hand, neurons in earlier auditory regions produce mainly a sustained, slowly adapting response, i.e. they respond for as long as the stimulus is present (Harms, Guinan, Sigalovsky, & Melcher, 2005). This suggests that the response in PT for moving stimuli possibly reflects adaptation of the phasic PT neurons to stationary sounds (Krumbholz,
Schonwiesner et al., 2005). The fact that motion processing takes place so late in the auditory pathway hierarchy, indicates that the auditory system analyses individual binaural representations of sounds in consecutive places in space, that are relayed from lower auditory regions rather than by creating a smooth continuous representation of auditory motion (Krumbholz, Schonwiesner et al., 2005).
Figure 2.8 (a) Axial and (b) sagittal view of the brain, showing activation related to processing of location cues (red monaural left/right vs diotic) and auditory motion (blue, moving
sounds>diotic) in the auditory cortex. Overlap is shown in yellow. Processing of location in c) IC and d) MGB (Krumbholz, Schonwiesner et al., 2005).
Other neuroimaging studies have also shown a response to moving sounds in PT (Baumgart et al., 1999; H.C. Hart et al., 2004; Krumbholz, Schonwiesner et al., 2005; Pavani et al., 2002; Warren et al., 2002) and there is some consensus that this is the motion centre of the auditory system, especially on the right (Baumgart et al., 1999). Krumbholz, Schönwiesner et al.(2005) have shown that the right PT responds to auditory motion in both hemifields, while the left responds only in the contralateral hemifield. This is consistent with right hemisphere dominance for spatial coding (c.f. literature on visual neglect). Note that, which particular anatomical field of PT is involved in motion processing is still not clear, as different studies appear to show variable
results. In terms of the anatomical regions identified by Rivier and Clarke (1997), sometimes response appears to be in medial region PA, while in other studies it appears to be in LA (Figure 2.6, left).
In the visual system, it has been suggested that there are two distinct stages of motion processing. The first stage takes place in the visual region V5, (Braddick et al., 2001; Ffytche, Skidmore, & Zeki, 1995) and the second, cognitive stage, in right parietal cortex (for a review see Battelli, Pascual- Leone, & Cavanagh, 2007). There is evidence for a similar scheme for the auditory motion processing. Warren and colleagues (2003) suggest that the PT reflects first stage, while parietal cortex reflects the second stage. Specifically, several studies suggest that the inferior parietal cortex, TPJ (Bremmer et al., 2001; Griffiths, Buchel, Frackowiak, & Patterson, 1998; Griffiths & Green, 1999; Griffiths et al., 2000; Griffiths, Rees et al., 1998; Krumbholz et al., 2007; Krumbholz, Schönwiesner et al., 2005) and operculum (Warren et al., 2002) are involved in aspects of motion processing. Furthermore, there is tentative evidence that neglect patients with a right parietal lesion show deficits in the perception of both static and moving spatial cues (Battelli et al., 2001). More studies are needed to confirm the link between neglect and a deficit in auditory motion processing. Additionally, there is evidence that the superior part of right parietal cortex is involved in auditory motion processing (Griffiths, Buchel et al., 1998; Griffiths & Green, 1999; Griffiths et al., 2000; Griffiths, Rees et al., 1998; Pavani et al., 2002), especially on the right. Lewald et al. (2002) showed that bilateral inhibition of the posterior parietal cortex with transcranial magnetic stimulation (TMS), shifted the perception of sound location, while it did not affect ITD discrimination acuity. This result indicates
that this region is involved in changes in spatial location, rather than the processing of spatial cues per se.
Poirier et al. (2005) have shown that moving sounds contrasted with static sounds induced a response not only in the right PT, premotor and parietal regions bilaterally, but additionally in visual motion regions V5. This is
tentative evidence to suggest that V5 is involved in motion processing of auditory stimuli, which is supported by a TMS study, which showed
impairment of auditory spatial judgment when TMS was applied in occipital cortex (Lewald, Meister, Weidemann, & Töpper, 2004).
In summary, auditory spatial coding appears to involve certain sub- cortical nuclei and the primary auditory cortex, while motion coding appears to involve PT. Parietal cortex appears to be involved in both static localization and motion processing. There is very little support from the animal literature of a motion-sensitive region, equivalent to PT (and visual V5/MT), which is partly due to the small number of motion-processing studies. Although posterior non-primary area PAF (and CM belt region in monkeys) appears to be sensitive to static location, there is no evidence that it is also sensitive to motion.
2.4 Summary
In this chapter, the coding of auditory features in the auditory system was discussed. There is considerable evidence from both human and animal studies that the main organizing principle of the auditory cortex is tonotopicity, which is particularly prevalent in primary auditory cortex. Furthermore, there is
evidence for regions sensitive to FM in antero-lateral non-primary auditory cortex, and regions sensitive to motion in posterior-medial non-primary auditory cortex. Note that this evidence comes mainly from human studies, possibly because these techniques are able to have an overview of the activity in the whole auditory cortex. This hierarchical organization of auditory cortical processing resembles very much the organization of the visual cortex. The question investigated in subsequent experimental chapters is whether selective attention to these features is also mediated in a way similar to the visual selective attention, i.e. in a feature-specific way.